A printing apparatus, for example, a so-called serial-type ink jet printing apparatus, comprises a carriage movable in the main scan direction which is mounted with a printing head capable of ejecting ink and an ink tank (liquid container) for containing the ink fed into the printing head. For printing an image, the printing apparatus repeats an operation of ejecting ink from an ejection port of the printing head toward a printing medium while moving the carriage in the main scan direction, and an operation of conveying the printing medium in a sub scan direction crossing to the main scan direction. The ink drops ejected from the printing head impact the printing medium to print the required image.
A mainstream ink used in such ink jet printing apparatus includes a dye as the colorant. In general, however, a dye ink somewhat lacks light fastness and gas fastness, and in some cases a print with the dye ink exhibits little image stability with durability in a special application such as for an outdoor notice.
In recent days, a printing apparatus using ink including a pigment as the colorant has been made available. A pigment ink has excellent light fastness and gas fastness, resulting in the print showing sufficient image stability. However, unlike the dye ink, the pigment ink needs to be handled with consideration for its dispersion properties.
The pigment molecules in the pigment ink do not dissolve in the ink solution as the dye molecules in the dye ink do, and are suspended and dispersed. If an ink tank containing the pigment ink is left still for a while, in some cases gravity makes the pigment particles gradually settle in the ink tank, giving rise to a concentration gradient of the pigment particles in the height direction of the ink tank. Specifically, a layer of a high colorant concentration is located in the bottom portion of the ink tank and a layer of a low colorant concentration is located in the upper portion thereof. When the ink is fed from the ink tank to the printing head in this situation to start and continue an operation of printing an image, a difference in concentration may possibly occur on the image between an initial stage and a later stage of the printing operation.
To give a concrete illustration, an ink jet printing apparatus structured to supply ink from the bottom of an ink tank to a printing head is considered. When the printing apparatus is installed with the aforementioned ink tank having a concentration gradient of the pigment particles and then starts the printing operation, the ink contained in the lower portion of the ink tank with a high colorant concentration is supplied in at an early stage in the printing, thus printing an image with a density higher than necessary. Thereafter, as the printing operation continues to be performed, the density of the printed image is gradually reduced along with the consumption of ink in the ink tank. Then, when the amount of the ink in the ink tank becomes low, the ink remaining in the ink tank has a lower colorant concentration than the original one. For this reason, even when the image is printed based on the same image data as that at the beginning of the printing operation, the printing density becomes lighter. Specifically, since a large diameter or a high specific gravity of the pigment particle makes the pigment particle significantly apt to settle, even when a non-use state of the ink tank lasts just for a few days, a concentration gradient as much as to affect the image may possibly occur.
The colorant concentration of the ink ejected from the printing head changes with the use of the ink tank as described above. This not only causes a difference in density of the printed image between the initial service stage and the later service stage of the ink tank. For example, in a color ink jet printing system using a plurality of color inks to express hues in predetermined color balance, the color balance may be lost. This event is to be recognized as a more remarkable problem of the difference between the image densities.
In order to maintain the colorant concentration of the ink drops ejected from the printing head within a constant concentration range regardless of the amount of ink remaining in the ink tank, the pigment molecules are desired to be uniformly dispersed in the ink tank, at least during the printing operation.
One proposal made for achieving such uniform dispersion of the pigment molecules is a structure for providing a stirring member in the ink tank for stirring the pigment molecules.
Japanese Patent Laid-Open No. 2005-066520 discloses an ink pack provided with a manually operable stirring member. The stirring member is shaped so as to be inserted into the ink pack from outside. An outwardly projecting part of the stirring member serves as a manipulating portion for operating a stirring section of the stirring member extending into the ink pack. Specifically, a user swings the stirring portion at regular intervals or as needed in order to stir the ink in the ink pack for dispersion of the pigment molecules.
Also, Japanese Patent Laid-Open No. 2005-066520 discloses an ink cartridge provided with a swinging member for stirring the ink in the tank through the use of an inertial force arising from the movement of the carriage during the printing operation, of which an example described is a stirring member formed integrally with the ink cartridge case. In this example, the stirring member extends in such a manner to hang down from the ceiling of the ink cartridge case toward the bottom, and has a cylindrically shaped weight formed at the lower end. An inertial force arising from an accelerating, stopping or reversal operation of the carriage makes the stirring member swing around the fulcrum, which is the pivoted end joined to the ceiling, in the moving direction of the carriage to stir the ink in the ink cartridge.
In addition, Japanese Patent Laid-Open No. 2005-066520 discloses another example of a stirring member which is not secured to the ink cartridge case and is movable freely over the bottom inner face of the ink cartridge. This stirring member is operative to move over the bottom inner face of the ink cartridge to stir the ink by the inertial force arising from the accelerating, stopping or reversal operation of the carriage.
Japanese Patent Laid-Open No. 2004-216761 discloses a stirring mechanism which comprises a shaft-shaped weight swinging in the right and left directions of the swinging central axis by the inertial force arising from the movement of the carriage, and a plurality of fins singing in the right and left directions in combination with the shaft-shaped weight. In this structure, since the plurality of the fins are arranged in parallel in the height direction of the ink cartridge, the ink is stirred equally from the upper portion to the lower portion of the ink cartridge.
However, neither of the foregoing Japanese Patent Laid-Open Nos. 2005-066520 and 2004-216761 have particular reference to a method for loading ink (liquid) into an ink cartridge (liquid container) provided with a stirring member.